Q. 28

(a) Liquids can be considered as very dense gases. When a liquid phase changes to gas phase, the liquid and the gas phases are in equilibrium and a surface separates the two phases. This surface is visible if both phases are in equilibrium and are below critical tempertaure and pressure. However, it is possible to interconvert liquid and gas wherein two phases are never present together.

With the help of a well-labelled diagram show that CO2 gas can be liquified by changing the pressure and temperature without passing through the situation when both gaseous and liquid CO2 are at equilibrium.

(b) Arrange the following liquids in increasing order of their viscosities. Give reason for your answer.

Water, benzene, ethane-1,2-diol.

Answer :

(a) Suppose gas is at point ‘A’ and temperature there is T1. Now, on increasing the temperature of the gas above critical temperature (Tc) keeping the volume constant, the gas will reach at point ‘F’ and consider there is temperature T2 and volume V1 with pressure p1.


Now on compressing the gas up to Volume V2. During this compression the pressure and volume of the gas will move along the curve FG and will reach at point G. Now, on


cooling the gas as soon as gas will reach the point ‘H’ located on isotherm of Tc, it will liquify without passing through equilibrium state. The gas will not pass through two phases because V2 of the gas is less than Vc i.e. molecules are closer to each other. Gas is at a higher pressure than critical pressure. Cooling slows down the molecular motion and intermolecular forces can hold the molecules together.



(b) The given liquids in increasing order of their viscosities are as follows-


Benzene < water < ethane-1,2 diol


Ethane-1, 2-diol shows more extent of hydrogen bonding than water while in benzene hydrogen bonding is absent. This factor of hydrogen bonding makes the liquids viscous.


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